When mounting a semiconductor device according to the flip-chip method on a substrate, for example a printed circuit board, electrical contact with the substrate is made using solder bumps. These solder bumps are formed on the semiconductor device and are reflowed after placing the semiconductor device on the substrate. The solder bumps are electrically connected with contact pads of an integrated circuit of the semiconductor device.
To promote adhesion of the solder bumps on the semiconductor device, the contact pads are provided with an under-bump metallization. This under-bump metallization also establishes a barrier against diffusion of components of the solder bumps into the pad metal and forms readily wettable surfaces for the solder bumps.
For correct functioning of the semiconductor device, a sufficient adhesion is required of the under-bump metallization on the contact pads. If adhesion is insufficient, the metallization can come loose from the pads as a result of mechanical loading, for example as a result of unintentionally dropping a product that contains the semiconductor device on a floor. If the semiconductor device does not function correctly, a number of process steps, such as the mounting itself, deposition of the solder bumps, and further packaging and assembly steps, may be carried out without benefit giving rise to unnecessary costs. In addition, a product including the semiconductor device can stop functioning correctly with many possible significant disadvantages. To assess adhesion of the under-bump metallization, a shear test is regularly applied using a monitor wafer, for example once in two to five batches a monitor wafer is used. A monitor wafer is also used for example when a change is made in the deposition process of the under-bump metallization, or for example when a change is made to a new type of wafer. The monitor wafer is provided with a relatively thick under-bump metallization layer, being for example three to five times thicker than an under-bump metallization layer on a regular wafer with semiconductor devices. A shear test is then carried out in which a yield strength and a failure mode of the under-bump metallization layer on the monitor wafer is determined.
From the result, the adhesion of the under-bump metallization on the regular wafers processed just before, just after and/or concurrently with the monitor wafer is assessed.
The shear test is destructive and gives an indirect assessment of adhesion of the under-bump metallization so its representativeness for regular wafers is uncertain.
It is an object of the invention to provide a semiconductor device provided with a test structure for assessing adhesion of under-bump metallization.